In the graphic arts industry, the use of laser scanners in the reproduction of color images is known. By means of the scanner, it is possible to separate an original color image (such as a photograph, a painting, etc.) into its yellow (y), magenta (m), cyan (c) and (optionally) black (k) components (i.e., its color separation images), and to store these as separate computer data files. By recombining these color separation images in a suitable output medium, a reproduction of the original image is obtained. To an increasing extent, original color images are generated on computers in the first place, but these also take the form of digital data files representing color separation images which ultimately must be combined in a suitable output medium.
If large numbers of copies of the image are required, it is necessary to generate a separate printing plate corresponding to each of the separation images and use them to print, in exact superposition, the separate y, m, c, k separation images using inks of the appropriate colors. The process can be performed repetitively at high speed to produce many thousands of copies. Before starting such an expensive process, however, it is customary to output the separation images in the form of a color proof which is a one-off color hardcopy that predicts, as accurately as possible, the appearance of the final printed image.
There are two distinct methods by which digitally-stored color separation images may be output as plates or proofs, namely "direct" and "indirect" methods.
The indirect method is the well-established technique whereby each of the stored color separation image files is output by laser scanning on to a separate black and white silver halide film which is developed and fixed in the normal manner. Each of the resulting black and white images is then used as a mask for the contact exposure of a plate or a single-color component of a photomechanical proof. Because both the plate and the proof are exposed through the same mask, and typically involve similar imaging chemistry (e.g. a photoresist), the proof provides an accurate simulation of the image that will ultimately be printed by the plate. The disadvantage of this method lies in the cost and inconvenience of using the intermediate films.
It has been recognized that "direct" laser exposure of plates and proofing materials represents a more cost-effective use of the technology and there are various disclosures of printing plates imageable by lasers. These embrace modifications to conventional plate coatings to enable laser address, as in the various photopolymer plates sensitised to Argon ion lasers and the adaptation of phenolic resin based plate coatings to infrared laser address, as disclosed in U.S. Pat. Nos. 5,340,699; 5,372,907; and 5,372,915, as well as "unconventional" imaging methods, such as laser thermal transfer.
In the laser thermal transfer method, a donor sheet comprising a layer of an infrared absorbing transfer medium is placed in contact with a receptor, and the assembly is exposed to a pattern of infrared (IR) radiation, normally from a scanning laser source. Absorption of the IR radiation causes a rapid build-up of heat in the exposed areas which in turn causes transfer of the medium from the donor to the receptor in those areas. Laser thermal transfer technology may be adapted for the production of lithographic printing plates by using a receptor having a hydrophilic surface (such as anodised aluminium foil) and transferring thereto a resinous, oleophilic material. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,401,606; 5,395,729; and 5,171,650; European Patent Application Nos. 0160396; and 0160395; and Japanese Publication No.04-140191. With a view to increasing the run-length of the resulting plates, the resinous oleophilic material is frequently designed to be crosslinkable, e.g. by heat treatment, and/or UV-irradiation, so as to enable the transferred image to be hardened in a separate post-transfer step. Such systems are disclosed, for example, in U.S. Pat. No. 5,395,729; EP Application Nos. 0160396; and 0160395. The crosslinking mechanisms disclosed include photocuring of unsaturated monomers and standard thermosetting processes, such as, the thermal curing of epoxy resins, phenol-formaldehyde resins, melamine-formaldehyde resins, etc.
Laser thermal transfer technology is also readily adaptable to the generation of color proofs. For this purpose, the transfer medium must comprise one or more dyes or pigments matching the color of one of the inks to be used in the printing process. The receptor is typically white paper or card, optionally bearing a colorless resin coating. All the separation images may be output in this fashion to a common receptor to provide a full color proof. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,171,650; and 5,126,760; WO Application No. 90/12342, Japanese Publications Nos. 63-319192; and 04-201485; and European Application Nos. 0542544; and 0602893.
Laser thermal transfer is potentially a highly attractive means for generating both proofs and plates directly from digitally-stored image data, since the use of photosensitive materials and wet processing is not required. However, matched proofing and platemaking media, imageable by the same or similar laser sources to provide well-matched proofs and plates, have not so far been described in the patent literature or made available commercially. Although certain patents, such as, U.S. Pat. Nos. 5,395,729; and 5,171,650; and WO Application No. 90/12342 disclose the generation of both proofs and plates by laser thermal transfer, there is no suggestion that identical donor media might be used for both purposes, or that any advantage would be gained from doing so. Indeed the examples in these patents invariably disclose the use of different donor media formulations for the respective imaging applications.
The state of the art teaches that different (and mutually exclusive) properties are required in transfer media intended for the separate imaging applications. For printing plates, the emphasis is typically on the durability of the transferred image and its ability to print many thousands of impressions without suffering undue wear. Hence, there is a tendency to use thicker transfer layers comprising tough, abrasion resistant resins, frequently hardenable by a post-transfer heat or UV treatment, but rarely containing dyes or pigments other than that required for absorption of the laser radiation (which is frequently a black-body absorber, such as, carbon black). In contrast, the important criteria for proofing media have been seen as the sensitivity of the imaging process, and the resolution and color fidelity of the transferred image, leading to the use of thin layers with a high pigment content, and the use of infrared-absorbing species that impart no visible coloration to the transferred image.
While these disparate approaches have been successful in optimising the performance of proofing and plate-forming media, judging each with respect to its own particular criteria, they make the correspondence between the proof and the plates (and ultimately the printed image) less accurate. Ideally, a given set of image signals, controlling a given laser imaging device, should produce the same results in both the proof and the plate, particularly with respect to parameters, such as, dot size, dot shape etc. Furthermore, any alteration to those signals (in pursuit, say, of a different color balance) should produce identical changes in both proof and plate. When transfer media with widely differing physico-chemical properties are used for the respective imaging applications, these goals become increasingly difficult to meet.